Cascade decays via intermediate states require the investigation of multiple-meson final states.
The simplest cases are pseudoscalar (PS) meson pairs like \(\pi \pi \)  or \(\pi \eta \) .
The reaction formalism and the sets of observables are discussed in [1]}, [2]} and
a field theoretic description of the process is given in [3]}. For single-meson production,
a `complete' experiment, which allows the unique determination of the magnitudes and phases of all relevant
amplitudes, requires the measurement of eight carefully chosen observables including single and double
polarization observables as a function of two kinematic parameters (typically center-of-momentum (cm)
energy and cm-polar angle) [4]}. Photoproduction of PS meson pairs requires
the measurement of eight observables as a function of five kinematic parameters to determine just the
magnitudes of the amplitudes, and 15 observables have to be measured to fix also their phases
[1]}.
